The invention pertains to a relief valve for turbines of exhaust gas turbochargers.
An exhaust gas turbocharger for an internal combustion engine usually contains a turbine and a compressor. The turbine is usually driven by the exhaust gas of the internal combustion engine, and the rotor of the turbine is connected in some way to the rotor of the compressor, as a result of which the rotation of the turbine rotor causes the compressor rotor to rotate also. The compressor then delivers combustion air under pressure to the associated internal combustion engine. A problem with turbochargers of this type is that the rpm's of the turbine rotor and thus of the compressor rotor increase with the rpm's and/or load of the internal combustion engine. At high operating rpm's or loads of the internal combustion engine, it is possible for the turbine and the compressor to be driven at excessive rpm's. It is also possible for the compressor to supply combustion air to the engine at pressures which are higher that the maximum allowable pressures for the machine.
Devices which go into effect when the rotor speed or load exceeds a certain value have already been installed in exhaust gas turbochargers. These devices usually have a relief valve, which allows at least some of the engine exhaust gas to bypass the turbine when the rpm's or load of the engine reaches a predetermined value. A relief valve of this type is usually designed as a poppet valve with a valve stem guided in a valve housing. The valve is actuated by the force of a spring and/or by a diaphragm, which forms the boundary of a pressure chamber and is actuated by compressed air.
A problem which occurs with the use of poppet valves is that the valve spring is exposed to the very high exhaust gas temperatures of the engine exhaust gas and thus becomes extremely hot. The heat flows along the valve stem and causes the valve stem and the valve housing to overheat. The direct conduction of heat via the valve stem and also the thermal radiation from the valve housing, for example, can cause the spring and the diaphragm to overheat. This overheating can cause the following:
the failure of the diaphragm and
a loss of stiffness of the spring, which leads to changes in the spring pretension and thus in the working point of the valve.
The prior art includes examples of how these problems are said to be prevented. Thus DE 30 09 453 C2 discloses a control device for the relief valves of exhaust gas turbochargers, in which the diaphragm is protected from thermal radiation by a radiation shield plate. This radiation shield plate is arranged in a space and divides this space into two chambers. A first chamber is bounded by the diaphragm and the shield plate, and a second chamber is bounded by the shield plate and the valve housing. Compressed air flows into the second chamber, flows around one side of the shield plate, and thus cools it. It then flows toward the valve disk along the valve stem through the gap formed between the valve stem and its guide in the valve housing, thus cooling the valve stem, and finally arrives in a space on the hot side, which is bounded by a hot-side radiation shield plate. From this hot-side space, the compressed air flows through a gap between the valve stem and the hot-side radiation shield plate and into the exhaust gas channel. The radiation shield plate protects the valve housing of the relief valve from the heat of the engine exhaust gas.
In this type of design for a control device for relief valves, the cooling effect on the diaphragm is not sufficient. The protection which the hot-side radiation shield plate provides from the hot engine exhaust gas in the exhaust gas channel is also low.
DE 35 09 019 C2 discloses a relief valve for a turbine of an exhaust gas turbocharger on an internal combustion engine. As also in the case of the above-cited DE 30 09 453 C2, a space in which a diaphragm is arranged is again provided, this space being divided by a radiation shield plate into a first chamber and a second chamber. Here, too, compressed air flows into the second chamber to cool the radiation shield plate from one side. The two chambers are connected to each other by an opening in the radiation shield plate. Here, too, the cooling effect leaves much to be desired.